There have recently been used honeycomb structures excellent in thermal resistance and corrosion resistance in various fields for diesel particulate filters (DPF) for trapping particulates exhausted from an diesel engine and catalyst carriers for loading a catalyst thereon for purifying automobile exhaust gas.
Such a honeycomb structure has a plurality of cells partitioned by porous partition walls and extending in the axial direction and can be manufactured by drying and firing a honeycomb formed body obtained by subjecting kneaded clay prepared by kneading a ceramic raw material powder, a binder, water, and the like, to a continuous extruder.
In a process for manufacturing the above honeycomb structure, inferiority such as deformation of cells or cracks is caused upon forming or drying and gives not a few inferior goods of undried or dried formed body recovered before finished goods are given. Therefore, it is preferable that a recovered material generated in a process for manufacturing the honeycomb structure and derived from a starting material for the honeycomb structure is reused as a recycled material from the view point of enhancing the recovery rate with respect to the raw material and inhibiting the yield of products from lowering.
There have conventionally known methods with regard to cordierite based honeycomb structures as methods for manufacturing a honeycomb structure using such a recycled raw material. In the case of manufacturing a new honeycomb structure by using, as a part of a starting material, a recycled raw material from a recovered material generated in a process for manufacturing a cordierite based honeycomb structure and derived from a starting material for the honeycomb structure, the resultant honeycomb structure has a larger thermal expansion coefficient in comparison with a cordierite based honeycomb structures produced by using an original starting raw material (raw material not containing a recycled raw material), and thereby the honeycomb structure tends to have lowered thermal impact resistance.
Therefore, in the case of applying the method to a product requiring high thermal impact resistance as a DPF or an exhaust gas purification catalyst carrier, a device of suppressing an increase in thermal expansion coefficient is necessary. For example, in a manufacture method described in the Patent Document 1, a product having low thermal expansion properties can be obtained by setting a cordierite reaction rate R (=protoenstatite/cordierite) to be 0.3 or less.
In addition, in a manufacture method described in the Patent Document 2, an unfired recycled raw material is pulverized to produce a pulverized material, pieces having a diameter of less than 1 mm are removed from the pulverized material, and a product is manufactured by using the rest of the pulverized material, thereby suppressing a rise in thermal expansion coefficient. Further, in a manufacture method described in the Patent Document 3, at least a part of the binder contained in the unfired recycled raw material is removed from the recycled raw material to make pulverization easier without deforming particles of the recycled raw material, thereby suppressing a rise in thermal expansion coefficient.
Patent Document 1: JP-B-3-72032
Patent Document 2: JP-A-119726
Patent Document 3: JP-A-302533